Displays using LEDs for illumination are becoming increasingly popular in commercial and residential environments. Digital billboards that can sequence through multiple advertisements are beginning to replace fixed signs along roadways and in front of businesses. Similarly, large video displays for instant replay are almost mandatory for large sports stadiums. LCD televisions are gaining market share from other technologies including plasma displays, and recently the Cold Cathode Fluorescent Lamp (CCFL) for LCD backlighting is being replaced by LED backlighting. Organic LEDs (OLED) displays, which were confined to research laboratories until very recently, are now being offered in commercial products.
LEDs are popular for such display applications due to the low cost, high energy efficiency, and long lifetime, however, variations in light output between individual LEDs and LEDs from each color component group limit performance and increase cost. For instance, the amount of light produced by an LED for a given current can vary by a factor of two to one or more between LEDs within a manufacturing lot and between lots, which when combined with the light produced by different color LEDs in a display pixel for instance, the blended color produced can vary tremendously. Likewise, the wavelength of the light produced by such LEDs can vary by 20 nm or more which produces a clearly visible color shift. Consequently, LED vendors typically sort LEDs into groups or bins with narrower specifications. LED customers may either purchase only specific bins for a higher price or design products that can tolerate wider tolerances, which may limit performance.
Further, an LED array that is designed and calibrated to produce uniform brightness and color when manufactured will degrade with use. As LEDs age, the light produced for a fixed current may increase or decrease over some amount of time and then will continue decreasing until end of life. Different color LEDs have different average aging characteristics, which may varying widely between individual LEDs. Consequently, perfectly built LED arrays will develop a grainy appearance with a different hue over time.
Displays with such arrays typically are recalibrated periodically over time to compensate for changing LED characteristics, which can be costly and time consuming. For instance, stadium displays produced by Daktronics, such as the one installed in the UT Austin football stadium, is apparently re-calibrated every two years, which takes a team of people roughly three days and nights. Special cameras with telescopes focus on each individual LED to determine brightness and color, which is fed back to the display controller. Billboards, which operate continuously, typically must be re-calibrated much more frequently to maintain optimum performance, which is costly and produces much downtime.
LED backlights for LCD televisions for instance produce white light from either phosphor coated blue LEDs or a combination of multi-color LEDs, such red, green, and blue. Such light typically passes through a diffusing layer before being applied to the back of the liquid crystal layer, which combines the light from the LEDs to produce uniform light behind the liquid crystal. Smaller displays typically have LEDs placed along one side of the display and inject light into specially shaped diffusing elements, while larger displays from Sony and Samsung for instance have arrays of LEDs behind the liquid crystal layer that match the physical dimensions of the display and typically use thin diffusing elements to produce uniform light from the LED point sources.
Displays with arrays of LEDs for backlighting have at least two advantages over display with LEDs along one or more sides. First, illumination generally is more uniform across the display, and second, the illumination from each LED in the array can be independently adjusted to improve the contrast ratio, which is called “local dimming” in the industry. However, illumination is more uniform only if the light output from each LED, or combination of LEDs for RGB backlights for instance, is the same. As in LED billboards and stadium displays, such arrays can be calibrated during manufacturing, but illumination uniformity and color, in particular with RGB backlights, will change over time for the reasons previously discussed.
LCD backlighting from multi-color LEDs, such as RGB, enables a display to produce a wider range or gamut of colors than backlighting from white LEDs, but is more difficult to control. LCDs with RGB backlights along one or more sides typically use three photo-sensors to detect the average intensity of each color component, which is fed back to the LED driver circuitry to maintain the proper mix of colors. Since the special light diffusing layer in such displays is effective at mixing the light from all the LEDs, the color across the entire display may be controlled using a single set of three photo-sensors. Although such photo-sensors and associated feedback circuitry is costly, the improved color gamut is sufficient to justify a higher price.
High end large screen LCD televisions with LED backlighting have recently been introduced by companies such as Samsung and Sony, which have arrays of LEDs that enable local dimming for high contrast ratios. At least some of such Samsung products have arrays of white LEDs, while at least some of such Sony products have arrays of RGB LEDs to support a wider color gamut. It is unclear how Sony maintains the proper color point, but such Sony products are sold for nearly twice the price of such Samsung products. The relatively simple approach described previously for RGB LED backlights along one or more sides of the display that uses one set of three photo-sensors to detect the intensity of each color component is not possible with such Sony products that support local dimming. Many sets of photo-sensors distributed throughout the array of RGB LEDs would typically be necessary to maintain uniform brightness and color across such a display.
A need exists for a means to maintain uniform brightness and color across an array of multi-color LEDs, such as RGB, used to directly produce images in the case of LED billboards and stadium displays, for instance, and indirectly in the case of LCD backlighting without the expense and complexity of on site pixel by pixel calibration in the case of LED billboards and stadium displays for instance, and special photo-sensors in the case of LCD backlights.